New imt string conversion routines.

[mhearne-usgs]

Check before merging:

• [ ] Style is acceptable style (e.g., variable names, comments, etc.)
• [ ] Docstrings are included for all methods with consistent formatting.
• [ ] Test coverage did not decrease.
• [ ] Is this PR likely to break downstream repositories? If so, check their tests.

[codecov-io]

Codecov Report

Merging #94 into master will decrease coverage by 0.62%. The diff coverage is 11.11%.

@@            Coverage Diff            @@
##           master     #94      +/-   ##
=========================================
- Coverage   95.52%   94.9%   -0.63%     
=========================================
  Files          29      30       +1     
  Lines        3622    3649      +27     
=========================================
+ Hits         3460    3463       +3     
- Misses        162     186      +24

Impacted Files	Coverage Δ
shakelib/utils/imt_string.py	0% <0%> (ø)
shakelib/utils/containers.py	86.16% <100%> (+0.26%)	:arrow_up:

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